The aim of this prospective descriptive study is to determine if exogenous surfactant (Exosurf) administration modifies the colloid osmotic pressure (COP) in preterm infants with hyaline membrane disease (HMD) and if these changes are related to respiratory function indices, pulmonary function tests and outcome. The hypothesis is that exogenous surfactant causes a rise in COP and that this rise correlates with improved respiratory indices and pulmonary function tests. Furthermore, the study will describe the normal COP in extremely low birth-weight infants (< 1250 g) with and without HMD. Status: Study completed.